1. Field of the Invention
The invention relates to electrical apparatus, and more particularly, to apparatus for protecting electrical circuits from damage due to ground faults.
2. Description of the Prior Art
In designing circuits for the transmission and distribution of electrical power, it is customary to provide means for protecting the conductors and electrical apparatus being powered from the conductors from overcurrent conditions. It has gradually been recognized that devices employed for overcurrent protection are not sufficient to prevent damage resulting from ground faults; that is, a short circuit between one or more of the electrical conductors and objects connected directly or indirectly to ground. The amount of current which flows through a ground fault may be below the level required to operate the overcurrent protective devices. However, this ground fault current can result in high transient overvoltages throughout the system, high temperatures due to arcing conditions leading to fire, or both.
Various methods of detecting and correcting ground faults have been used in the past. A current transformer is positioned to surround the conductors of the circuit being protected. This transformer, also referred to as a current monitor, produces an output signal upon occurrence of a ground fault condition at a point downstream from the current monitor on the circuit being protected. The monitor is connected to a separate device known as a sensor which includes a switching device such as a relay actuated by the output from the current monitor when a ground fault occurs. The contacts of the relay are then used to interrupt the supply of electrical power to the circuit being protected. A device disclosed in copending U.S. Patent Application Ser. No. 696,326, filed June 15, 1976, by John T. Wilson performs the same function and is self-powered and self-contained in a unitary molded insulating housing. It would be desirable to provide a self-powered self-contained ground fault device which is suitable for use at a variety of ground fault trip current levels.